This disclosure relates to hydraulically damped elastomeric powertrain mounts deployed to control shake and vibration responses resulting from various sources such as road inputs. The hydraulically damped powertrain mount commonly comes in the form of a rubber isolator containing a hydraulic fluid cavity which acts as a pump when excited by vibration. The pumped fluid oscillates through a tube called an inertia track which creates a fluid resonance system and associated damping to dissipate the vibration energy and reduce the shake response. These types of mounts are generally shown and described in commonly owned published international patent application WO105768 A1, the entire disclosure of which is hereby expressly incorporated herein by reference.
It is often desired to have a “bypass valve” designed into the fluid system to allow the pumped fluid to bypass the inertia track and flow directly into a low pressure reservoir under certain conditions, typically low amplitude vibrations, such as engine idle speed inputs, where isolation rather than damping is preferred to eliminate vibration. This bypass valve usually takes the form of a simple rubber disc or other flat shape, fitted between two perforated rigid forms, such as pierced metal plates, which is positioned to provide a short, direct route to the low pressure reservoir.
Clearance between the thickness of the disc and the spacing of the perforated plates permits unimpeded flow between the pump chamber and low pressure reservoir for small vibration inputs, while effectively sealing the pathway and forcing the fluid to flow through the inertia track during high amplitude vibration. The disc or other shape that redirects fluid flow depending on the amplitude of the vibration input is referred to as a decoupler, and forms a key component in most hydraulically damped powertrain mounts made today.
A common problem with this type of arrangement is noise that is generated by the decoupler, commonly referred to as decoupler “chortle”. Under high amplitude inputs, the decoupler component is forced back and forth between the upper and lower plates under significant hydraulic pressure and generates noise as it contacts each plate. The volume and character of this internally generated noise is dependent on several design factors such as the area of the decoupler surface that contact the upper and lower plates, pumping capacity of the mount assembly as a function of vibration input, and spacing between the plates. Usually decoupler rattle/chortle is a problem at mount transients (large displacements) such as engine start-up/shutdown. Further, mounts with relatively large decoupler travels (i.e. small surface area) are prone to exhibit the problem.
Historically, this self-generated noise is undetectable within the passenger compartment. However, with the improvements in sound quality in automobiles over the years, decoupler chortle continues to be a noise and vibration handling (NVH) issue that automotive engineers often struggle with.